Tumor hypoxia is observed in many cancer types. It is believed to be a consequence of unorganized growth of new vasculature. Cancer cells close to the blood vessels have high oxygen consumption due to rapid proliferation. Cells that are farther away are masked from oxygen supply. These hypoxic cancer cells are mostly in a phase of cell cycle arrest and therefore become highly resistant to chemotherapy and radiation therapy which mostly affects dividing cells. Different therapy techniques have been studied to target hypoxic tumors. To predict the efficacy of radiation therapy for a cancer patient and to select optimal therapy strategy it is essential to assess oxygen distribution in the tissue. However, none of the existing methods for assessing tissue oxygenation has yet been established in the clinical arena. In this R21 application we propose to develop a non-invasive, high resolution imaging method for tissue oxygen. Based on our recent development of photoacoustic probing of oxygen sensitive dye's lifetime, we propose a technique that combines the accuracy and sensitivity of time-resolved fluorescence methods with the high resolution and deep penetration of photoacoustic imaging. It has the potential to fill the gap for a fast, easy to use, real-time imaging of tumor hypoxia. This research proposal addresses the basic aspects of developing this new imaging method. First, we will build a prototype system that will be tested on phantoms. This would be our prime vehicle for optimizing the system hardware design and software algorithms. Then we will test the system and compare different dye administration methods on small animals. These results will serve as a basis for a continuing research to translate it into a clinical tool. Our vision for the ultimate outcome of this project is a non-invasive clinical imaging modality for better prognosis and treatment decision making in most cases of head and neck cancers (where tumor is accessible to light penetration). For deeper tumor locations (e.g. breast cancer tumors) invasive optical fiber illumination will be considered. Such imaging modality could have a profound impact on clinical practice in oncology and particularly in radiation oncology.